It is common practice to cool certain types of radiation detectors to cryogenic temperatures where high precision is required, particularly the semiconductor detectors of infrared radiation used in infrared spectroscopy. Cooling of the detector to a very low temperature reduces the effects of thermal noise on the detector output signal. To maintain the detector device at both a relatively low and substantially constant temperature, it is thermally isolated from the ambient environment by insulation (usually including a vacuum chamber) and is cooled by a low temperature cooling agent, commonly liquid nitrogen, although other liquified gases (e.g., helium) may be used depending on the temperature at which the detector will be maintained.
The most common type of cryogenically cooled detector structure currently used for infrared spectroscopy includes a Dewar in which the inner and outer vessels forming the Dewar are cylindrical and are constructed of aluminum. The inner vessel is suspended from the top of the outer vessel by a short, thick, fiberglass-epoxy tube which is cemented at its junctions with the inner and outer vessels with epoxy resin. The tube provides thermal isolation between the inner and outer vessels but permits liquid cooling agent to be poured into the inner vessel through a hole in the top of the outer vessel. Since the fiberglass tube also insulates the inner vessel electrically from the outer vessel, an electrical lead wire is attached to the inner vessel and extends through an opening in the outer vessel to a terminal which can be connected to ground to maintain the inner vessel at ground potential. The infrared detector device, e.g., a mercury cadmium telluride (MCT) semiconductor, is mounted to the cylindrical outer surface of the inner vessel so that heat from the detector can be transferred directly to the relatively cool wall of the inner vessel. Radiation is admitted to the detector through a window mounted in the cylindrical side wall of the outer vessel. Typically, this window is held in place by a custom formed copper fitting and an elastomer O-ring engaged to the fitting to seal the space between the inner and outer vessels from the ambient atmosphere.
To further reduce transfer of heat to the inner vessel and the detector, the inner vessel is usually wrapped by many layers of aluminized polyester such as Mylar TM film, commonly known as "superinsulation". After the placement of the super insulating polyester such as Mylar TM film, about the inner vessel, the inner vessel is mounted within the outer vessel, the region between the vessels is evacuated to a low pressure and then sealed to maintain the vacuum. Typically, small charcoal pellets (called "getters") are embedded in epoxy resin on the top of the inner vessel and function to absorb gases which would otherwise accumulate within the vacuum region.
In addition to any gases that may enter the vacuum region by diffusion or leakage from the outside atmosphere, it has been found that materials within the vacuum region will expel gases by a process known as "outgassing". In particular, the low temperature epoxy which is used to cement the fiberglass tube in place is a significant source of these gases. This low temperature epoxy has a relatively high outgassing rate, but even the metal surfaces within the vacuum chamber exhibit outgassing although at a substantially lower rate. In addition, while the layers of mylar superinsulation serve to reduce heat transfer by minimizing the infrared radiation reaching the inner vessel, they interfere with the rapid establishment of a satisfactory low pressure within the vacuum chamber since the layers of mylar tend to trap gas molecules which gradually escape by diffusing between or through the film layers.